Due to lockdown settings or Polkit policy, shutdown may not be
available. If this is the case, the "Power off ..." action should
be hidden from the user status menu.
https://bugzilla.gnome.org/show_bug.cgi?id=652038
Around 2.91.90, gnome-session-save was renamed to gnome-session-quit.
This commit restores compatibility with the older gnome-session, for
those testing under GNOME 2.32 or below, by calling the DBus methods
directly.
https://bugzilla.gnome.org/show_bug.cgi?id=644591
The DBus JS binding will complain if the signature of a method
or of a signal is undefined, so we need to define it even if it
is an empty string, and we need to use the correct property name
for signals.
https://bugzilla.gnome.org/show_bug.cgi?id=643374
